Polyurethane flexible foams are well recognized articles of commerce which have applications in numerous diverse fields. Flexible polyurethane foams are prepared by the reaction of a di- or polyisocyanate with an isocyanate reactive component in the presence of a suitable urethane reaction-promoting catalyst, a chemical or physical blowing agent, and a suitable foam stabilizing surfactant. Although much early research in the area of polyurethane foams involved the use of isocyanate-terminated prepolymers and quasi-prepolymers, the bulk of polyurethane flexible foam produced commercially is prepared by the so-called "one-shot" technique. The "one-shot" process was developed due to its flexibility and the inability to prepare molded foams by the prepolymer process. Although the "one-shot" process requires the foam manufacturer to inventory a variety of different ingredients, the process remains the predominant polyurethane foam technology today.
In the "one-shot" technique, two or more separate streams of isocyanate, polyol, catalyst, blowing agents, etc., or combinations thereof, are introduced into a mix head. The reactive mixture exiting the mix head may be allowed to rise freely, generally on a lined conveyor, to produce slab foam, or may be poured or injected into molds to produce molded foam. Slab foam may be sliced to thicknesses suitable for carpet underlay, seat cushions, and the like. Molded foam allows complex contours to be produced, for example for use in automotive seating. Due to the differences between the free rise associated with slab foam and the constrained rise of molded foam, the polyurethane reactive ingredients are tailored to the specific application.
While physical blowing agents such as methylene chloride, chlorofluorocarbons (CFC's) and low boiling alkanes have been used as physical blowing agents, their use has been severely curtailed by environmental concerns, particularly the Montreal Protocol. High resilience polyurethane flexible foams have employed water as a reactive blowing agent for many years. Water reacts with a portion of the isocyanate groups present to generate an amine and carbon dioxide. The amine reacts with additional isocyanate to produce urea linkages, while the carbon dioxide provides the necessary blowing effect.
The urea groups generated in high resilience foam tend to harden the foam. The fraction of urea groups as compared to urethane groups in high resilience foam in the nominal 2-4 lb/ft.sup.3 density range is small enough so that the increased hardness can be compensated by suitable choice of the polyurethane reactive ingredients, particularly polyols and crosslinkers. However, as the density decreases below the 2 lb/ft.sup.3 range, increased levels of water and isocyanate necessary to achieve lower density result in unacceptable hardness due to the increased urea group content. To lower the hardness, physical blowing agents such as methylene chloride have been added. Methylene chloride appears to exert a plasticizing effect, increasing foam softness. However, the use of methylene chloride is environmentally undesirable.
There is a long-standing need for exceptionally soft, very low density polyurethane flexible foams, for example those having densities of 1.5 lb/ft.sup.3 or less, and 25% indentation load deflections (ILD) of less than 15 lbs. Such soft, low density foams can be used as replacements for soft cushioning material such as polyester fiberfill, and for other uses as well.
In copending U.S. application Ser. No. 08/311,378 is disclosed a one-shot formulation suitable for hypersoft, low density polyurethane flexible foams. While the formulations disclosed therein produce a foam with the desired density and softness, a prepolymer process is not disclosed.
Prepolymer and quasi-prepolymer technology has been used in the past to prepare free rise high resilience slab foam employing water as a reactive blowing agent. However, attempts to lower density while at the same time affording a very soft foam have not been successful. In general, as with one-shot water-blown foams, the increased amount of water used as a blowing agent increases the urea-group content of the foam. Low density foams prepared from such formulations have been too firm for use in cushioning applications. Prepolymer techniques have seldom been employed for high resilience molded foam.
R. E. Knox, "Molding of Prepolymer Based Resilient Urethane Foam," RUBBER WORLD, 139, 1959, pp. 685-692 discloses the use of isocyanate-terminated prepolymers admixed with additional toluene diisocyanate, reacted with water at 5% above the stoichiometric equivalent. Despite the presence of c.a. 5% didecylphthalate plasticizer, the 2 to 2.5 lb/ft.sup.3 foams exhibited ILDs which were still in the range of 20 lbs. Moreover, rather than reduce part weight by further density reduction, Knox proposes core molding instead, attesting to the problems associated with low density, all-water blown prepolymer foam.
J. H. Sanders and K. C. Frisch in POLYURETHANES, CHEMISTRYAND TECHNOLOGY, Part II, "Technology," Interscience Publishers, N.Y., in Chapter VII, "Flexible Foams" discusses the use of water in conjunction with isocyanate-terminated prepolymers to produce flexible polyurethane foams. However, this treatise indicates that a large excess of water will use up the free isocyanate groups, reducing the isocyanate available for effective cross-linking. Although foam density decreases with increasing water content, when 30-50% water in excess of stoichiometry is used, physical properties are said to markedly decrease, and therefore only a 10-20% excess is used to foam prepolymers. A 1.3 lb/ft.sup.3 one-shot (not prepolymer) foam is disclosed on page 65 of the reference. However, even with the addition of 10 parts CFC-11 per 100 parts polyol, foam hardness is quite high, in the range of 24-30 lbs.
In U.S. Pat. No. 5,070,114, free rise and molded foam employing specific prepolymers is disclosed. The prepolymers are prepared by reacting a polyol component, preferably one with from 10 to 25% by weight oxyethylene residues, with a particular blend of MDI isomers containing from 2 to 40% of 2,4'-MDI. One example of a foam with a free rise density of 1.25 lb/ft.sup.3 is exemplified, prepared from a prepolymer derived from the reaction of an isophoronediamine-modified, urea-containing isocyanate with a polyol having 14% ethylene oxide moieties, the prepolymer having an NCO group content of 11.3%. However, the resulting foam was firm and in addition had low elongation and tear strength, the latter being but 1 lb/in.
G. F. Lunardon et al., "Production of Soft Block Foams and TDI-Based Cold Cure-Molded Foams With No Use of CFCs", 32ND ANNUAL POLYURETHANE TECHNICAL/MARKETING CONFERENCE, Oct. 1-4, 1989, discloses use of specialized TDI-derived prepolymers including both a conventional polyol and a polyol with high ethylene oxide content to produce water-blown polyurethane foam. The prepolymers have little polyol content, the NCO group content ranging from 38.5 to 42.5 weight percent, and the range of water used is conventional, resulting in foams in the 3 lb/ft.sup.3 range.
All water-blown prepolymer foams having densities of less than 1.5 lb/ft.sup.3 and simultaneously having a 25% ILD of less than 15 lbs have not previously been prepared. It would be desirable to provide a process for their preparation, and compositions suitable for use therein. It would be further desirable to prepare soft, low density foams having the aforementioned characteristics which provide acceptable tear strength and elongation.